1. Field of the Invention
The present invention relates to an active-matrix-type display device, and more particularly to a display device which has a pixel-memory with a high aperture ratio and high definition.
2. Description of the Related Art
A liquid crystal display device of a TFT (Thin Film Transistor) type which includes a switching element in each pixel portion has been popularly used as a display device of a personal computer or the like. Further, the TFT-type display device is also used as a display device of a portable information device such as a mobile phone. The display device used in the a portable information device has been required, compared to the conventional liquid crystal display device, to satisfy the further miniaturization and the further reduction of power consumption.
In using a battery or the like as a power source of the display device, it is necessary to reduce the power consumption which is brought about by the display. Accordingly, there has been proposed an idea to impart a memory function to each pixel of the conventional liquid crystal display device.
Japanese Patent Laid-open 2003-302946 (USP 7057596) discloses a liquid crystal display device in which a pixel includes two pairs of transistors for holding a video signal and an additional capacitor which is connected to a pixel electrode. A stored charge of an additional capacitor makes image signal which is written into the pixel electrode.